This invention relates to detecting the symbol boundary in orthogonal frequency division multiplexing (OFDM) signals, such as IEEE 802.11 for a Wireless Local Area Network (WLAN) OFDM signals.
OFDM systems use multi narrow-band sub-channels for improved tolerance to multipath delay. The sub-channels are orthogonal to each other to prevent inter-carrier interference (ICI). One problem with OFDM systems is that they can be very sensitive to synchronization errors. A wrong estimation of a symbol boundary in OFDM leads to increased inter symbol interference (ISI) and ICI, which degrades the performance of the OFDM system. Thus it is desirable to provide an accurate estimation of a symbol boundary.
A WLAN OFDM data packet has a standard field structure. As an example, the field structure of a packet used in 802.11a/g is shown in FIG. 1. The data packet 100 includes a short training field (STF) 101, a long training field (LTF) 102, a signal (SIG) field 103 and a rest of packet (ROP) field 104.
The STF 101 includes a set of ten identical short preambles S1, S2, . . . , S10, each having a duration of 0.8 μs. The STF 101 is followed by the LTF 102. The LTF 102 includes a double guard interval (DGI) and two identical long preambles L1 and L2. DGI is of 1.6 μs duration. The DGI is followed by the two identical long preambles L1 and L2, each having a time duration of 3.2 μs. The samples of the DGI are a copy of the samples in the last 1.6 μs duration of L1 or L2. The LTF 102 is followed by the SIG field 103, which includes L-SIG (non-high throughput (non-HT) signal field) for 802.11a/g, or L-SIG (non-HT signal field) and HT signal field (HT-SIG) for 802.11n HT transmission, or HT-SIG for 802.11n GreenField transmission, or L-SIG and very high throughput signal field (VHT-SIG-A) for 802.11 ac VHT transmission.
The ROP field 104 may include additional STF and LTFs and signal fields depending on the type of transmission. The ROP field 104 includes the payload data, which is represented as data symbols of 3.2 μs duration. Each data symbol is preceded by samples of guard interval duration from the end of the data symbol. Guard interval (GI) of 0.8 μs (long GI) or 0.4 μs (short GI) is used in WLAN OFDM transmission.
For extracting payload data from the data packet 100, the correct boundary of OFDM symbols must be known. The symbol boundary can be found using the known information of the STF and the LTF of the received packet. Traditionally, two methodologies have been used to find the symbol boundary. The first method finds the boundary between S10 and DGI (the S10-DGI boundary), and the second one finds the boundary between DGI and L1 (the DGI-L1 boundary).
The S10-DGI boundary is usually found using an auto-correlation scheme. However, due to high noise and low Signal to Noise Ratios (SNRs), the boundary position determined using such schemes is offset from the correct boundary due to the poor correlation metric of short preambles at low SNR. Thus, the timing variance of the auto-correlation synchronization scheme is large and may degrade performance of the OFDM system. Accordingly, such auto-correlation synchronization schemes are used to achieve only a coarse estimate of the S10-DGI boundary.
The DGI-L1 boundary can be found using a fine symbol boundary (FSB) algorithm, such as the FSB algorithm in Schmidl, T. M. and Cox, D. C., “Robust Frequency and Timing Synchronization for OFDM”, IEEE Trans. Commun., vol. 45, no. 12, pp. 1613-1621, December 1997, which uses the auto-correlation based on the long preambles. However, this algorithm suffers from high latency, low accuracy for low SNRs and does not cover the precursors of the channel.